Process for stabilizing particulate alkali metal percarbonate

ABSTRACT

The invention relates to a process for stabilizing particulate alkali metal percarbonate by coating by coating it with an aqueous solution, preferably concentrated, of a complex formed from a boric acid or borate and an organic diol, polyol or hydroxycarboxylic acid, to the resulting coated product, and to compositions including the product. The coating can additionally contain an alkali metal neutral salt such as sodium chloride or sodium sulphate.

BACKGROUND OF THE INVENTION

The present invention relates to a process for stabilising particulatealkali metal percarbonate, and particularly to a coating processtherefor, to the particulate percarbonate having improved stability soproduced and to washing or bleaching compositions containing the same.

It is well known that alkali metal percarbonates, including sodiumpercarbonate in particular, can be used as bleaching compounds indetergent powder mixtures, e.g. for domestic clothes washing. Incomparison with alkali metal perborate tetrahydrate, they have theadvantage of dissolving more rapidly at 20° C., which is becomingincreasily beneficial in view of the trend towards lower washingtemperatures. Another advantage of percarbonates is that they areenvironmentally friendly. However, percarbonates have a knowndisadvantage of decomposing more quickly than sodium perboratetetrahydrate during storage in the powdered state, particularly ifstored in a damp atmosphere. Moreover, other constituents of washingcompositions can accelerate their decomposition.

In order to improve the stability of percarbonates, and particularlysodium percarbonate, it has been proposed to bring the percarbonateparticles into contact with various stabilising materials, and inparticular to coat the percarbonate with those materials. Organic and/orpolymer compounds such as paraffins, polyols, vinyl resins etc. andinorganic compounds such as silicates, borates, perborates, boric acidsetc. have been proposed as coating materials.

A French patent to Kao Corporation published under number 2 528 447describes coating the surface of sodium percarbonate with sodium borate.A process for coating sodium percarbonate consists of wetting the sodiumpercarbonate with water, mixing the damp sodium percarbonate with sodiumborate in powder form and then drying them at a temperature above themelting temperature of the sodium borate used. This process requires asubstantial amount of energy because of the high temperature required tomelt sodium borates. In addition, the fact that the sodium percarbonateis wetted requires strict control of its water content to prevent orminimise decomposition.

In British patent 1 575 792, to Interox SA there is described a processfor stabilising sodium percarbonate by coating it with a solution of aboric acid. We have found that coatings with boric acid can be veryeffective at stabilising percarbonate, but the solubility of boric acidsin water is somewhat limited, particularly at ambient temperatures. Inconsequence, it is necessary to use rather larger volumes of coatingsolution than for more soluble coating materials, which isdisadvantageous because it would be desirable to find a way ofincreasing the solubility. Such low concentrations are disadvantageousfor industrial use, because more thermal energy is needed to dry thepercarbonate particles after coating, thereby increasing the dryingcosts, and the prolonged drying periods can cause percarbonatedecomposition, reducing the value of the eventual product.

It is an object of the present invention to provide a process forstabilising percarbonate particles which ameliorates or overcomes atleast some of the disadvantages of the above-mentioned processes and/orprovides an alternative to the above-described processes,

BRIEF SUMMARY OF THE INVENTION

The present invention provides a process for stabilising a particulatealkali metal percarbonate by coating it with an effective amount of aboric acid or borate containing coating material, characterised in thatthe coating material comprises a complex of boric acid or a borate withan organic diol polyol or hydroxycarboxylic acid.

DESCRIPTION OF PREFERRED EMBODIMENTS

Herein by the term effective amount is meant an amount such that therate of decomposition of percabonate is reduced, when it is stored in anatmosphere at 80% relative humidity and 40° C. in the presence of 10%w/w zeolite 4A.

Without being bound by any theory, it is believed that the effectivenessof coatings obtained in the present process may be attributable to thenature of the complexes of the boric acid and/or borate with the di orpolyol on the surface of the percarbonate, enabling them to spreadacross the surface of the percarbonate and thereby form a barrier thatprevents or reduces interaction between the percarbonate core and theenvironment, be it water vapour in the atmosphere and/or otherparticulate materials in for example detergent compositions in whichpercarbonate is incorporated as a bleach.

The coating solutions of the present invention can be obtained byintroduction of preformed complexes between the oxyboron compound andthe diol, polyol or hydroxycarboxylic acid or by the introduction ofboth components into the coating solution enabling the complex to formin situ. The complexes of the present invention can contain or bederived from any boric acid or alkali borate. The precise nature of theoxy-boron species in solution prior to the formation of complexes willdepend on the pH of that solution and the extent to which the solutionis able to equilibrate. Suitable boric acids for introduction into thecoating solution include ortho and metaboric acids and suitable boratesthat can be introduced into solution include martials containingtrigonal BO₃ or tetrahedral BO₄ groups or both arranged in either ringanions or chain anions. In solution, the borate species are or becomehydrated. Expressed empirically, useable borates include those offormula MB₅ O₈ and M₂ B₄ O₇ and M₃ B₃ O₆, in which M represents analkali metal or ammonium ion, and preferably either sodium or potassium,such borates typically being hydrated in solution. Viable boratesinclude, metaborate, tetraborate, pentaborate and octaborate, typicallyhydrated at the time of of after introduction into solution. Also, theremay be contemplated use of a perborate of empirical formula M₂ B₂ O₆.nH₂O in the coating solution to supply at least a fraction of the borate.In many instances, a single boron compound is introduced into thecoating solution, though mixtures may be used, and indeed in the coatingsolution, a range of oxyboron species is often generated.

The second essential component of the coating complex comprises a diol,polyol or hydroxycarboxylic acid. Such compounds are characterised bythe presence of at least two groups within the second component that arecapable of complexing with boron at the same time, ie the component is abidentate or polydentate ligand, at least one of which groups is anhydroxyl group and the other or others may be either hydroxyl orcarboxylic acid groups. The alcohols that can form complexes herein canbe either aliphatic (including alicyclic) or aromatic, and thehydroxycarboxylic acids are normally aliphatic. It will be understoodthat the hydroxy-containing complexing compounds may be monomeric oralternatively can be polymeric provided that such polymers retainpendant hydroxyl and where appropriate carboxylic acid groups. It willbe further recognised that where the complexing compound is liquid atambient temperatures, it is preferable to employ an excess of the boricacid or borate above the stoichiometric amount to complex with theorganic complexing compound, so as to avoid the presence of any liquidagent of the percarbonate surface.

The complexing compounds can suitably be selected from the followingclasses of compound:

aliphatic and alicyclic polyhydric alcohols;

sugars and alcohols derived therefrom;

oligosaccharides and polysaccharides;

aromatic polyols;

aliphatic hydroxy carboxylic acids as such or as a salt,

vitamins and enzymes;

Aliphatic polymers with pendant hydroxyl and carboxylic acid groups.

It will be recognised that there is some degree of overlap betweencertain of the classes above, as for example between polyols and sugarderivatives and between aliphatic hydroxycarboxylic acids and thevitamin/enzyme classes and some examples of the one class are alsoexamples of the other class as well. A mixture of two or more diols,polyols or hydroxycarboxylic acids may be employed, selected either fromthe same class or from different classes listed above.

Examples of compounds in the classes are as follows:

aliphatic and alicyclic polyhydric alcohols such as pentaerythitol, andsugars and alcohols derived therefrom include galactose, fructose,xylose, mannitol and sorbitol;

oligosaccharides and polysaccharides include mannan, galactomannan and βlactose;

aromatic polyols include naphthol, gallic acid and dihydroxybenzoicacid;

aliphatic hydroxy carboxylic acids or salts, preferably water-soluble,such as

ammonium or particularly an alkali metal, especially sodium salt,include citric acid, tartaric acid, gluconic acid, saccharic acid andlactobionic acid and sodium salts thereof.

Vitamim and enzymes include acscorbic acid and riboflavin;

Polymers include polyvinylalcohol and polyhydroxyacrylic acid or thesoluble eg sodium salt thereof, known as PHAS, each preferably having anaverage molecular weight in the range of 30,000 to 200,000;

The proportion of boron compound in the complex is generally from 10 to90% by weight calculated as H₃ BO₃, based on the total weight of thecomplex, is often at least 20% w/w and in many instances is from 50% to80% by weight of the complex. The diol, polyol or hydroxycarboxlic acidconstitutes the balance of the complex.

Various of the complexes employed in the present invention and theirformation can be represented as follows, where L represents a bidentateligand such as tartrate: ##STR1##

As it can be seen, in the resultant complexes, the ligand is bonded viatwo oxygen bridges to the boron and the compound is no longer either aboric acid or a borate.

The coating agent of the present invention is advantageously employed inthe form of an aqueous solution. In practice, the concentration of thecoating complex, ie the total of boron compound and diol, polyol orhydroxycarboxlic acid, in the aqueous coating solution is at least halfof, and preferably as close as is convenient to, its saturationconcentration of the solution at the application temperature. In thisway, only a comparatively small and preferably the smallest practicablequantity of water needs to be evaporated subsequently to produce drypercarbonate particles, thereby requiring a lesser or least heat input.It will be recognised that the invention complex has a significanthigher solubility than boric acid. The concentration of coating agent inthe aqueous coating solution is generally at least 15% by weight,preferably at least 20% by weight. Concentrations above or equal toapproximately 25% by weight are particularly advantageous.

The dissolution of the complex or its components can take placeconveniently at a temperature of from 15° to 95° C., and preferably from20° to 70° C.

In addition to the foregoing essential components of the complex, thecoating agent may contain a proportion of one or more compounds that areknown to ameliorate decomposition of percarbonates when employed as acoating, or possibly as a mixture with percarbonates. It will berecognised that where such optional components are employed in thepercarbonate coating, they may be employed in a separate coating, forexample a second solution added before, simultaneously with or after thesolution of the borocomplex of the present invention.

Among the optional components, specific mention is made of thosematerials which have hitherto been suggested as stabilising coatings forpercarbonate or for other persalts, including particularly silicates,phosphates, and chelating agents selected from metal polycarboxylicacids and polyphosphonic acids. Such materials may be employed as thesole optional component or mixtures of optional components can be used.

Such silicates are often selected from alkali metal silicates having amole ratio of soda:silica of from 2:1 to 1:4 and in accordance with thedisclosure in co-pending British Patent Application 9226796 the amountof silicate and the soda:silica ratio are selected in conjuction withother materials employed in the same aqueous solution such that in usethe solution does not gel. Such phosphates can be ortho, meta orpolyphosphates and are often in alkali metal salt form, e.g. sodium orpotassium, including both fully and partially neutralised salts. Suchchelating agents that are suitable optional components herein oftencomprise aminopolyalkylcarboxylic acids and/oraminopolyalkylenephosphonic acids which obey the formula ##STR2## inwhich M represents either --CH₂ --CO₂ H or --CH₂ --PO₃ H, x representsan integer selected from 1 to 6, and preferably is 2, and y representsan integer selected from 0, 1, 2 or 3. Within this general formulaespecially preferred stabilisers include ethylenediamine tetra aceticacid (EDTA), ethylenediamine tetrakis (methylenephosphonic acid)(EDTMP), and diethylenetriamine pentakis (methylenephosphonic acid)(DTPMP). An alternative and highly effective complexing agent comprisescyclohexane-1,2-tetramethylene phosphonic acid.

The amount of the optional components is often at the discretion of theuser. A convenient amount to employ of the above optional component orin total of the optional components is sometimes in the range of fromabout 0.1% to about 10% w/w of the percarbonate, in many instances lessthan 5% w/w and in certain instances from about 0.25 to about 2% w/w.

In addition to the borocomplex, it is possible as an option to includein the coating agent solution one or more neutral alkali metal orammonium salts, and in particular a halide such as chloride, and/orsulphate and/or nitrate salts. In a number of attractive coatings, theneutral salt comprises a mixture of a sulphate and a chloride.Preferably, the alkali metal is either sodium and/or potassium, but maycomprise any of the other alkali metals such as lithium. Particularlydesirable neutral salts include sodium chloride, potassium chloride,sodium sulphate and potassium sulphate and any two or more in mixture.If present, the neutral salt provides the benefit of increasing thethickness of coating on the percarbonate particles for a specifiedamount of boron-containing compound, and by virtue of the solubility ofthe salts, can often enable such a thicker coating to be achievedwithout increasing the volume of water that needs subsequently to beevaporated away in the drying stage, or at least not increasing thevolume of such water pro rata.

In mixtures containing the borocomplex and the neutral salt, theborocomplex often provides at least 20% w/w of the mixture and in anumber of attractive mixtures from 30 to 70% w/w of the mixture.However, the proportion of borocomplex chosen usually also takes intoaccount the total weight of coating applied to the percarbonate and thepreferred minimum weight of borocomplex in the coating, as indicatedsubsequently herein. The borocomplex for use in the mixture can bechosen at the discretion of the user from any of the borocomplexesdisclosed hereinbefore.

In a number of preferred embodiments, the proportion of the organicconstituent of the complex is less than 45% by weight of the entirecoating, including both the complex and the neutral alkali metal salt,if present, and particularly from 10 to 30%.

In yet further options, the user can contemplate the incorporation of aneutral alkaline earth metal salt such as a magnesium or calciumsulphate and/or chloride, preferably in a minor proportion and oftenfrom 0 to 20% w/w of the mixture with borocomplex, because the presenceof such alkaline earth metal salts can tend to depress the solubility ofboron compounds in aqueous solution.

The quantity of coating agent used, including both essential componentsand optional components, usually represents 0.5 to 20% w/w of the coatedpercarbonate. Preferably, the quantity is selected in the range of from1 to 15% w/w and in many instances from 2 to 10% of the coatedpercarbonate. As a general indication, when a coating is applied in thesame way, the extent to which percarbonate decomposition is amelioratedincreases as the thickness of the coat increases, though non-linearly.The selected weight of coating takes into account the manner of coating,the length of the period for which the resultant composition shouldremain stable and the environment in which the percarbonate will beused, such as the temperature and humidity of storage conditions and theproportion of relatively aggressive washing composition constituentslike zeolites.

In some embodiments, the percarbonate particles are coated with arelatively small quantity of coating agent, such as from about 2% w/w toabout 6% w/w, which ensures that the percarbonate particles initiallycan enjoy a high active oxygen content, which is maintained by thecoating. In a number of such embodiments, the weight of boric acidapplied is often in the region of about 0.5 to 1% w/w, the weight oforganic component in the complex is often from about 0.4 to about 0.8%w/w and the weight of neutral salt such as sodium chloride is often fromabout 1 to 2.5%.

The alkali metal percarbonate is preferably sodium percarbonate. In manyof the coated percarbonate products produced according to the presentinvention, the bulk density of the percarbonate so coated is generally0.8 to 1.2 kg/l. The rate of dissolution of percarbonate that has beenso coated is often acceptably rapid, when determined according tointernational standard ISO 3123-1976. The time corresponding to a 90%dissolution of the sample of percarbonate coated according to theinvention does generally not exceed 2.5 minutes.

The particulate percarbonate that is suitable for coating in any processaccording to the present invention can have been prepared in any processknown as such for making an alkali metal percarbonate, e.g. by thedirect method, by fluid bed processes, or the so-called wet processes inwhich percarbonate is crystallised from a saturated aqueous solutions,often by cooling and/or by addition of an alkali metal salt.

The percarbonate core particles which are coated according by a processto the present invention can incorporate various additives in a widerange of proportions and in accordance with known teachings and/orpractice. Such additives include, amongst others, persalt stabilisers,crystal habit modifiers and salting out agents.

Persalt stabilisers can be selected from one or more of alkali metal andalkaline earth metal silicates, alkali metal and alkaline earthphosphates, magnesium compounds such as magnesium sulphate, chloride oroxide, organic complexing carboxylic acids and their salts, such asethylene diamine tetraacetic acid and/or salt, ordiethylenetriaminepentaacetic acid and/or salt and/or organicpolyphosphonate complexing agents such ashydroxyethylidenediphosphonate, andalkyleneaminopolymethylenephosphonates, includingethylenediaminetetramethylenephosphonic acid and/or salt,diethylenetriaminepentamethylenephosphonic acid and/or saltcyclohexane-1,2-diaminetetramethylenephosphonic acid and/or salt.

In some highly desirable embodiments, the process of the presentinvention is employed to coat sodium percarbonate that has been producedby a manufacturing/stabilising process described either in GB-A-1 553505, published in the name of Interox Chemicals Limited or in GB-A-1 578062, published in the name of Peroxid-Chemie GmbH, in both of which thepercarbonate contains a small amount of up to about 0.5% w/w silicatedistributed within its particles and a further small amount of up toabout 0.5% of a silicate or silicate derivative on the surface of itsparticles, as a result of the timing and distribution of the addition ofsilicate in two stages during the crystallisation and recovery of theparticulate percarbonate.

Crystal habit modifiers act on the morphology of the percarbonatecrystals and include organic polymeric compounds like polyacrylates andinorganic species such as polyphosphates e.g. sodium hexametaphosphate.

Salting out agents are used during the crystallisation of thepercarbonate from solution, and typically are highly water-solublealkali metal salts, such as sodium chloride, sodium sulphate, sodiumhexametaphosphate etc.

It will be understood that some agents can simultaneously provide anumber of different properties, such as stabilisation and crystal habitmodification.

The average diameter of the percarbonate core particles that can becoated by a process according to the present invention is generally atleast 100 μm and often not greater than 2000 μm, and in many instances,the average particle size falls in the range of 250 to 1000 μm, such ascommercially available percarbonate having an average particle size ofabout 500 μm or about 550 μm. The spread of percarbonate particles is atthe discretion of the percarbonate producer. As a practical matter, andas has been realised for many years, it is advantageous forincorporation in particulate detergent compositions to avoid very fineparticles, such as particles of below about 100 or 150 μm, because suchparticles are inherently more susceptible to environment-induceddecomposition--they present a much higher surface area to volume ratiothan larger particles. For many particulate detergent compositions, itis also desirable to avoid or minimise particles larger than about 1500μm, to reduce potential problems of persalt/detergent particlesegregation. Thus, in many practical embodiments of the presentinvention all or substantially all the percarbonate to be coated canpass through a sieve of 1500 μm and be retained on a sieve of 150 μm,and in some of which or other embodiments at least 80% w/w is retainedon a sieve of 350 μm and passes through a sieve of 1000 μm.

The process according to the present invention by which percarbonateparticles are coated with the coating agent described above can compriseany method known in itself for contacting persalts with coating agent. Apreferred means for bringing the coating agent into contact with thepercarbonate comprises spraying an aqueous solution of the coating agentonto the percarbonate particles. It is particularly desirable for thepercarbonate particles to be kept in motion. Thus, a coating process ofthe present invention can desirably be carried out in a range ofapparatuses that can agitate particles, of which practical examplesinclude a fluid bed, a rotating plate, and a rotary mixer into each ofwhich it is convenient to spray the coating agent solution. During thecourse of the contact, the persalt tends to adsorb, and to some extentabsorb the coating agent solution and with subsequent or simultaneousevaporation of the solvent from the coating agent solution, a coating isdeposited around the percarbonate core.

It will be recognised that the invention coating process may beconducted in a single pass throught the coating apparatus or in aplurality of passes, at the discretion of the user. A plurality ofpasses is particularly beneficial for application of a heavy coating inthat it reduces the amount of solvent that need be removed in each passand thus reduces or removes the risk of over-wetting the percarbonatebefore it is dried. A continuous or batch method can be used.

Evaporation of solvent from the solution can be carried out at the sametime and in the same vessel as spraying. The two steps can alternativelybe carried out separately in different apparatus, which may in somecases be of the same type, e.g. both in fluidised beds or be ofdifferent types, such as the mixing step in a rotary mixer and theevaporation step in a fluidised bed piece of equipment.

An apparatus such as a fluid bed is particularly suitable for carryingout simultaneous spraying and evaporation. In such a case, thetemperature of the fluid bed is usually maintained in the range of 30°to 95° C. and preferably 60° to 80° C.

One particularly advantageous process variation comprises contacting acharge of the percarbonate particles with a solution of the inventioncoating agent in a separate mixer, particularly a rotary mixer, anddrying the wetted percarbonate subsequently in a fluid bed. The solutioncan be introduced into the mixer by spraying or even via a coarse spraysuch as one or more nozzles. In this separate mixer variation, thetemperature in the mixer is often selected in the range of 10° to 60° C.and preferably 20° to 50° C. Drying in the fluid bed is then oftencarried out at a temperature of 50° to 90° C. and preferably of from 60°to 70° C.

The fluid bed employed herein either for a combined coating/dryingprocess or simply in the drying stage can be operated in accordance withknown procedures for coating/drying or simply drying persalts, as thecase may be. Thus, any non-reactive gas can be used as the fluidisinggas, including air in particular. The gas can be pre-dehumidified, ifdesired, and pre-heated to maintain the temperature of the fluid bed atthe desired value. It is also possible to use direct heating means forthe fluidised bed, such as a tube bundle placed within the fluid bed ora heated jacket around the bed. The upward airflow of fluidising gas isregulated to maintain the percarbonate particles in an agitated state,ie not settling, but is not so great as to blow the particles, otherthan fines, out of the fluidising vessel.

The aqueous coating solution is generally brought into contact with thepercarbonate particles at a temperature that is within about 30° C. ofthe particles and often within about 5° C. of each other.

The proportions of coating agent solution and percarbonate are chosen soas to leave, after drying, the desired weight of coating agent aroundthe percarbonate core. In practice, it is desirable to limit theaddition of solution to percarbonate in a fluid bed or mixer to amaximum water content of about 18% w/w so as to minimise or eliminatewetting out problems, more preferably to an amount selected in the rangeof about 5 to 12% w/w water and often from about 8 to 12% w/w water. Itis normally desirable also to continue drying until the coatedpercarbonate has a moisture content of below about 1% w/w, such as inthe region of 0.1 to 0.7% w/w. The duration of the drying stage isusually determined by such practical considerations as, amongst others,the amount of coating agent solution being applied per unit weight ofpercarbonate, the residual content of moisture that will be tolerated,the temperature and moisture content of the influent fluidising gas,whether additional heating is employed for the bed and the rate at whichthe gas flows through the bed. It will accordingly vary from apparatusto apparatus and be capable of control by a skilled person in the art ofcoating persalts with the aid of preliminary ranging trials.

It will naturally be recognised that the final form of a coating agenton percarbonate may change as a result of subsequent reaction orprocessing. Thus, for example, contact of a solution of an acidiccoating agent on the surface or in the surface layer of percarbonate, analkali, as in the aforementioned British Patent Specification 1 575 792,may result in the interaction of the acid with the alkali, and that thedrying stage can release water of hydration from some or all hydratedsalts that might be expected to be present at ambient temperature,provided that the drying temperature exceeds the transition temperaturefor such salts.

The present invention also relates to washing or bleaching compositionscontaining particulate coated sodium percarbonate, such as productaccording to the present invention described hereinabove and/or producedby the process according to the present invention hereinabove.

In many preferred compositions according to the present invention, oneor more of the composition components are selected within the followingnarrower bands:

    ______________________________________                                        percarbonate   2 to 40%, particularly 5 to 30%                                surfactant     2 to 40%, particularly 5 to 25%                                builder        1 to 60%, particularly 5 to 40%                                diluent        1 to 70%, particularly 5 to 50%                                additives      1 to 10% in total.                                             ______________________________________                                    

The surfactants for incorporation in solid compositions of the presentinvention can be selected from particulate or flaky anionic, cationic,non-ionic, zwitterionic, amphoteric and ampholytic surfactants and canbe either natural soaps or synthetic. A number of suitable surfactantsare described in chapter 2 of Synthetic Detergents by A. Davidsohn andB. M. Milwidsky (6th edition) published in 1978 by George Godwin Ltd andJohn Wiley & Sons, incorporated herein by reference. Without limiting tothese surfactants, representative sub-classes of anionic surfactants arecarboxylic acid soaps, alkyl aryl sulphonates, olefin sulphonates,linear alkane sulphonates, hydroxy-alkane sulphonates, long chain andOXO alcohol sulphates, sulphated glycerides, sulphated ethers,sulpho-succinates, alkane sulphonates, phosphate esters, sucrose estersand anionic fluorosurfactants; representative classes of cationicsurfactants include quaternary ammonium or quaternary pyridinium saltscontaining at least one hydrophobic alkyl or aralkyl group,representative classes of nonionic surfactants include condensates of along chain alkanol with either polyethylene oxides or with phenols, orcondensates of long chain carboxylic acids or amines or amides withpolyethylene oxide, and related compounds in which the long chain moietyis condensed with an aliphatic polyol such as sorbitol or condensationproducts of ethylene and propylene oxides or fatty acid alkanolamidesand fatty acid amine oxides; representative classes ofamphoteric/zwitterionic surfactants include sulphonium and phophoniumsurfactants, optionally substituted by an anionic solubilising group.The proportion of surfactant, expressed as a fraction of all thesurfactant present is often from 2/10 to 8/10ths anionic, from 0 to6/10ths nonionic, and from 0 to 3/10ths for the other surfactants.

Detergent builders that are suitable for inclusion in compositionsaccording to the present invention include specifically alkali metalphosphates, particularly tripolyphosphate but also tetrapyrophosphateand hexametaphosphate, especially the sodium salt of each, alkali metal,preferably, sodium carbonate, alkali metal, preferably, sodium borates,and siliceous builders including clays like bentonite, zeolites such asX, Y and MAP zeolites (EP-A-0 552 053) and layered silicates such as theproduct available under the trade designation SKS6. The coatingsachievable with the boric acid-containing agents of the presentinvention render sodium percarbonate that has been so coated at higherlevels particularly suited to incorporation in the relative aggressivedetergent compositions, ie those containing siliceous builders. Usefuldetergent compositions can also include organic chelating buildersinclude nitrilotrisodium triacetate (NTA), EDTA, EDTMP and DTPMP, Suchchelating builders can be employed in a relatively small amount as anaugmenting builder and peroxygen stabiliser, such as of 1 to 10%.

The detergent compositions can also contain diluents, in an amountusually of not more than about 50% w/w. Such diluents include sodium andmagnesium sulphate and are less favoured than previously bymanufacturers of detergent compositions, who in recent years havepromoted concentrated compositions.

Detergent compositions of the present invention can also contain othersubstances selected for dedicated purposes in detergent compositions,which in some instances are referred to collectively as detergentadditives. Among such additives, the following can be mentioned: persaltactivators, optical brighteners, foam inhibitors, enzymes, fadinginhibitors and anti-redeposition agents, colorants, pH regulators. Suchadditives for incorporation in persalt-containing detergent compositionshave been described in greater detail in Chapter 4 and exemplified inChapter 7 of the aforementioned work by Davidsohn and Mildwidsky and arewell known to skilled practitioners. Thus, for example, the bleachactivator is typically a compound which generates a peroxyacid or ananion thereof by reaction with the percarbonate and is employed in amole ratio of about 4:1 to 1:2 percarbonate:activator for monoactivatingactivators and proportionately for multiactivating activators. The rangeof activators a1 to a20 described by Solvay Interox Ltd in EP-A 0 565017 can be employed herein, including TAED, SNOBS, sodiumisononoyloxybenzenesulphonate, TAGU or sugar esters. Another type ofactivator for washing/bleaching compositions comprises certaintransition metal salts and/or complexes, for example certain manganese,cobalt, and titanium complexes, sometimes employed in conjunction with acalcium promoter, as described in European Patent Application-A-0 272030. Commonly used optical brighteners include stilbene derivatives.Commonly used optical brighteners include stilbene derivatives. Commonantiredeposition agents include carboxymethyl cellulose and polyvinylpyrrolidone.

The washing and/or bleaching compositions can be employed for washingand or bleaching operations, such as for domestic laundry in accordancewith currently described operation conditions for respectively persaltor persalt plus activator-containing compositions.

Although described herein with reference to alkali metal percarbonates,the coating agents and coating solutions herein containing a borocomplexcan likewise be contemplated in a modification of the present inventionfor use with and to stabilise other peroxygen compounds which couldbenefit from enhanced stability, for example if they being consideredfor incorporation in particulate detergent compositions. Such peroxygencompounds include other persalts (materials which generate hydrogenperoxide in aqueous solution), typically alkali metal and especiallysodium salts, including especially persalts that are less stable thansodium perborate tetrahydrate, including perphosphates and persilicates.

Certain embodiments of the present invention are described hereinafterin greater detail by way of example only.

EXAMPLES 1-4 AND COMPARISONS C5 AND C6

In each of these Examples, particulate sodium percarbonate (1 kg)(commercially available from Solvay Interox under its trade mark OXYPERand having a weight average particle size of 480 μm was coated withcomplexes formed from boric acid and hydroxycarboxylic acids.

A solution of a borocomplex coating agent in Example 1-3 was prepared bydissolving orthoboric acid, 150 g, and hydroxycarboxylic acid salt, 50g, in demineralised water (DMW, 1 liter) and adjusted to pH7 at atemperature of about 20°-30° C. In Example 1, the salt comprisedtrisodium citrate, in Example 2, potassium D-saccharic acid and inExample 3, disodium tartrate.

The particulate sodium percarbonate was charged into a laboratory scalefluidised bed drier (available under the trade mark AEROMATIC),fluidised with hot air and heated to a bed temperature of 70° C. Afraction of the previously prepared coating solution, 263 mls, wassprayed onto the fluidised bed in a substantially constant manner over aperiod of 45 minutes. The bed continued to be fluidised for a furtherperiod of about 5 minutes to ensure that the coated percarbonateparticles had been dried and the resultant material bore a coating of 5%w/w of the borocomplex.

In Example 4, the coating solution comprised 99.8 g of a solution of pH6obtained by dissolving o-boric acid (25 g), PLAC (a polylactone)hydrolising to poly α hydroxyacrylate, (12.72 g, av mol wt of 170,000)and NaOH (7.28 g), providing a coating of 3% w/w on the percarbonate.The coating procedure comprised first pouring the coating solution ontoa further 1 kg sample of the commercial particulate sodium percarbonatethat was being agitated in a Logide M5R mixer, and thereafter drying thedamp percarbonate for 45 minutes at a bed temperature of 70° C. in anAEROMATIC fluid bed drier.

In Comparison C5, the procedure of Examples 1-3 was followed, exceptthat the coating solution comprised 100 g orthoboric acid in 1 liter ofDMW that was heated to between 50° and 60° C. until the solute wasdissolved of which 310 mls solution was sprayed into the bed to providea dry weight coating of 3% w/w. In Comparison C6, C5 was followed, butusing 526 mls of solution sprayed onto the fluidised bed over a periodof 60 minutes to provide a dry weight coating of 5 % w/w. It wasobserved during repeat runs of C5 and C6 that the coating solutiontended to block the sprayhead periodically unless remedial heating ofthe spray and periodic flushing was carried out.

The solubility and stability of the products of Examples 1-4 were testedusing the procedures outlined below:

The rate of dissolution of the percarbonate was measured byInternational Standard method ISO 3123-1976. The proportion ofpercarbonate dissolving after 60 seconds is given.

The suitability of coated percarbonate for bulk storage was tested bymeasuring its heat output using an LKB isothermal microcalorimeter. Thesuitability of the coated percarbonate for incorporation with otheringredients in a washing or bleaching composition was determined by dryblending the coated sodium percarbonate particles (15% w/w) with aparticulate detergent base composition (85%) which contained carbonateand Zeolite 4A builders, packing the resultantant blend in glasscontainers and storing them in controlled temperature and humidityconditions of 40° C. and 80% Relative Humidity, and measuring theavailable oxygen content of the composition at intervals to determine,by comparison with the original content, the proportion remaining. Thesame base detergent was employed for testing the products of Examples1-3 and C5-C7, but a different and more aggressive base detergent (90%w/w)was employed to test the product of Example 4 (10% w/w).

The coated percarbonate was further compared with the uncoated feedstockpercarbonate (C7).

The results of the tests are summarised in Table 1 below. For Ex 4, the% solubility is after 2 minutes and the Avox stability after 4 weeks isquoted and are differentiated in Table 1 by the presence of an *.

                  TABLE 1                                                         ______________________________________                                                           Bulk storage Avox Stability                                Ex. % Dissolved after                                                                            suitabiity - LKB                                                                           % remaining after                             No  1 minute at 15° C.                                                                    μW/g 40° C., 16 hr                                                               6 weeks                                       ______________________________________                                        1   96             7.4          80                                            2   84             9.0          76                                            3   93             7.5          78                                            4    83*           5.8           79*                                          C5  84             12.7         75                                            C6  84             14.1         74                                            C7  89             5.8          39                                            ______________________________________                                    

From Table 1, it can be seen that coated products according to thepresent invention had similar or better properties than either uncoatedpercarbonate feedstock or products coated with the corresponding boricacid by itself. In particular it will be observed that for two of theinvention products the solubility of the coated product (Ex1 and Ex3)was actually superior to uncoated material (C7), and the Example coatedproducts had a lower heat emission than the coated comparison productsC5 and C6 indicating that they were safer to store in bulk beforeincorporation into a detergent composition. Thirdly, it will berecognised that all the coated products were markedly superior to theuncoated feedstock material. Moreover, the invention products wereobtained using higher concentration solutions thereby requiring lessevaporation of water from the coating solution, and without encounteringthe sprayhead blocking problems observed when the lower concentrationcomparison products were made.

EXAMPLES 1a TO 1i

The process of Example 1 can be repeated employing respectively sorbitol(1a), mannitol (1b), gluconic acid/Na or K salt (1c, 1d), lactobionicacid/Na or K salt (1e,1f) or the corresponding K salts of the saltsemployed in Ex1, Ex2 or Ex3 (1 g, 1h, 1i) instead of trisodium citrateof Ex1.

EXAMPLES 8 TO 13

In these Examples, the process of Example 4 was repeated at the 1 kgscale, using the coating solutions S1 to S4 as described in Table 2below, gravity fed through a tube over 5 minutes onto the particulatepercarbonate in the mixer, but employing a subsequent drying period inthe fluidised bed of 30 minutes. In the solutions described in Table 2,the proportions of the constituents are given by weight. The extent ofcoating and the properties of the coated sodium percarbonate are shownin Table 3, in which the % dissolution, bulk storage and avox stabilityfigures were obtained in the same way and under the same conditions asthose quoted in Table 1.

                  TABLE 2                                                         ______________________________________                                        Solution                  Tartaric                                                                             Sodium                                       Ref    Boric acid  NaCl   acid   Gluconate                                                                            Water                                 ______________________________________                                        S1     2           3      1             18                                    S2     5           12     3             60                                    S3     3           8             4      40                                    S4     5           12            3      60                                    ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                                % w/w % Dis- Bulk  Avox                               Example                                                                              Solution                                                                              Amount   coating                                                                             solution                                                                             Storage                                                                             left %                             No     Ref     g        w/w   1 min  μW/g                                                                             6 weeks                            ______________________________________                                        8      S1      124      3      95    5.0   77                                 9      S1       82      2      85    4.6   68                                 10     S2      124      3     100    4.4   76                                 11     S2       82      2     100    0.8   79                                 12     S3      113      3     100    --    65                                 13     S4       82      2     100    --    67                                 ______________________________________                                    

By way of comparison, the avox remaining in a sample of the uncoatedfeed stock sodium percarbonate after the same storage period and in thesame base detergent composition was only 39%.

From Table 3, it can be seen that the coated products were substantiallymore storage stable in a detergent composition than was the feedstockmaterial, and that the compositions containing tartaric acid within thecomplex were on average more stable than when the complex containedgluconate. An intermediate storage stability is obtained when apolyhydroxyacrylate is substituted in the same weight proportion forrespectively the tartaric acid or sodium gluconate in the solutionsidentified above, though it tended to have a higher bulk heat emissionthan would be preferred.

We claim:
 1. A process for stabilizing a particulate alkali metalpercarbonate by coating it with an effective amount of boric acid orborate containing coating material, wherein the coating materialcomprises a complex of boric acid or a borate with an organic diol, apolyol, a hydroxycarboxylic acid, or a water soluble salt of ahydroxycarboxylic acid.
 2. A process according to claim 1 wherein thealkali metal percarbonate comprises sodium percarbonate.
 3. A processaccording to claim 1 wherein the complex is formed from a boric acid. 4.A process according to claim 3 wherein the complex is formed bydissolving boric acid and an hydroxycarboxylic acid and/or a solublesalt thereof in water at a temperature of from ambient to 60° C.
 5. Aprocess according to claim 1 wherein the complex is formed from borax.6. A process according to claim 1 wherein the complex is formed from analiphatic hydroxycarboxylic acid or soluble salt thereof.
 7. A processaccording to claim 6 wherein the aliphatic hydroxycarboxylic acid orsalt thereof employed to form the complex is selected from citric acid,tartaric acid, gluconic acid, saccharic acid and lactobionic acid andthe alkali metal salts thereof.
 8. A process according to claim 7wherein the complex is formed from orthoboric acid and sodium citrate orsodium tartrate.
 9. A process according to claim 1 wherein the complexcontains 50 to 80% by weight of boric acid or borate, calculated as H₃BO₃ based on the total weight of the complex.
 10. A process according toclaim 1 wherein the concentration of the complex in the aqueous solutionof coating material brought into contact with the percarbonate particlesis at least 15% by weight.
 11. A process according to claim 1 whereinthe coating material additionally comprises a neutral alkali metal orammonium salt.
 12. A process according to claim 1 wherein the coatingmaterial is used in a quantity corresponding to 0.5 to 20% by weight,based on the weight of the coated alkali metal percarbonate.
 13. Aprocess according to claim 12 wherein the coating material comprises atleast 1% w/w of said complex, calculated as HBO₃.
 14. A processaccording to claim 1 wherein coating is carried out by spraying theaqueous solution of the coating material onto the percarbonateparticles.
 15. A process according to claim 14 wherein the percarbonateparticles are agitated in a fluid bed during coating.
 16. A processaccording to claim 15 wherein the temperature of the fluid bed is 30° to95° C.
 17. Process according to claim 14 wherein the percarbonateparticles are coated with the coating solution in a mixer and then driedin a fluid bed.
 18. Process according to claim 17 wherein thetemperature in the mixer is from 10° to 60° C. and wherein thetemperature of the fluid bed is from 50° to 90°.
 19. Coated alkali metalpercarbonate particles obtained by a process according to claim 4, 10,14, 15, 16, 17 or
 18. 20. Particulate alkali metal percarbonate havingimproved stability by coating with an effective amount of a boric acidor borate containing coating material, wherein the coating materialcomprises a complex of boric acid or a borate with an organic diol, apolyol, a hydroxycarboxylic acid, or a water soluble salt of ahydroxycarboxylic acid.
 21. A product according to claim 20 wherein thealkali metal percarbonate comprises sodium percarbonate.
 22. A productaccording to claim 20, wherein the complex is formed from a boric acid.23. A product according to claim 20, wherein the complex is formed fromborax.
 24. A product according to claim 20, wherein the complex isformed from an aliphatic hydroxycarboxylic acid or soluble salt thereof.25. A product according to claim 24 wherein the aliphatichydroxycarboxylic acid or salt thereof employed to form the complex isselected from citric acid, tartaric acid, gluconic acid, saccharic acidand lactobionic acid and the alkali metal salts thereof.
 26. A productaccording to claim 25 wherein the complex is formed from orthoboric acidand sodium citrate or sodium tartrate.
 27. A product according to claim25, wherein the complex contains 50 to 80% by weight of boric acid orborate, calculated as H₃ BO₃ based on the total weight of the complex.28. A product according to claim 25 wherein the complex is formed fromboric acid and an hydroxycarboxylic acid and/or a soluble salt thereof.29. A product according to claim 20, wherein the coating materialadditionally comprises a neutral alkali metal or ammonium salt.
 30. Aproduct according to claim 20, wherein the amount of the coating is 0.5to 20% by weight, based on the weight of the coated alkali metalpercarbonate.
 31. A product according to claim 30, wherein the coatingcomprises at least 1% w/w of said complex, calculated as HBO₃.
 32. Awashing or bleaching composition containing at least one washing agentand a coated particulate alkali metal percarbonate having improvedstability by being coated with an effective amount of a boric acid orborate containing coating material, wherein the coating materialcomprises a complex of boric acid or a borate with an organic diol, apolyol, a hydroxycarboxylic acid, or a water soluble salt of ahydroxycarboxylic acid.
 33. A composition according to claim 32, whereinthe alkali metal percarbonate comprises sodium percarbonate.
 34. Acomposition according to claim 32, wherein the complex is formed from aboric acid.
 35. A composition according to claim 32, wherein the complexis formed from borax.
 36. A composition according to claim 32, whereinthe complex is formed from an aliphatic hydroxycarboxylic acid orsoluble salt thereof.
 37. A composition according to claim 36, whereinthe aliphatic hydroxycarboxylic acid or salt thereof employed to formthe complex is selected from citric acid, tartaric acid, gluconic acid,saccharic acid and lactobionic acid and the alkali metal salts thereof.38. A composition according to claim 37, wherein the complex is formedfrom orthoboric acid and sodium citrate or sodium tartrate.
 39. Acomposition according to claim 32, wherein the complex contains 50 to80% by weight of boric acid or borate, calculated as H₃ BO₃ based on thetotal weight of the complex.
 40. A composition according to claim 32,wherein the coating additionally comprises a neutral alkali metal orammonium salt.
 41. A composition according to claim 32, wherein theamount of the coating is 0.5 to 20% by weight, based on the weight ofthe coated alkali metal percarbonate.
 42. A composition according toclaim 41, wherein the coating comprises at least 1% w/w of said complex,calculated as HBO₃.